In our studies we have identified insulin and lactogen receptors in rat liver Golgi vesicles and have demonstrated that they are strikingly similar to those in the plasma membrane. We hypothesize that they are precursors for those in the plasma membrane but do not exclude the possibility that they play some kind of intracellular role. We are evaluating the role of these receptors as precursors for those in the plasma membrane by: (1) Studying the time course of induction of the rat hepatic lactogen receptor in both Golgi fractions and plasmalemma and (2) Evaluating the pattern of decrease of the rat hepatic lactogen receptor following inhibition of receptor production. We are studying the role of these intracellular receptors as possible sites of hormone action by seeing if the intact hormone can associate with the intracellular sites. To this end we are injecting 125I-labeled hormones in vivo and determining the time course of association of the label with subcellular structures by both radioautography and cell fractionation procedures.